Component stacking using pre-formed adhesive films

ABSTRACT

A method of forming integrated circuits includes laminating a patterned film including an opening onto a wafer, wherein a bottom die in the wafer is exposed through the opening. A top die is placed into the opening. The top die fits into the opening with substantially no gap between the patterned film and the top die. The top die is then bonded onto the bottom die, followed by curing the patterned film.

TECHNICAL FIELD

This invention relates generally to integrated circuit manufacturing processes, and more particularly to methods for bonding semiconductor dies onto wafers.

BACKGROUND

With the evolution of semiconductor technologies, semiconductor dies are becoming increasingly smaller. However, more functions need to be integrated into the semiconductor dies. Accordingly, the semiconductor dies need to have increasingly greater numbers of I/O pads packed into smaller areas, and the density of the I/O pads rises quickly. As a result, the packaging of the semiconductor dies becomes more difficult, which adversely affects the yield.

Package technologies can be divided into two categories. One category is typically referred to as wafer level package (WLP), wherein dies on a wafer are packaged before they are sawed. WLP technology has some advantages, such as greater throughput and lower cost. Further, less under-fill and/or molding compound is needed. However, WLP suffers from drawbacks. As aforementioned, the sizes of the dies are becoming increasingly smaller, and the conventional WLP can only be fan-in type packages, in which the I/O pads of each die are limited to a region directly over the surface of the respective die. With the limited areas of the dies, the number of the I/O pads is limited due to the limitation of the pitch of the I/O pads. For example, if the pitch of the I/O pads is to be decreased, solder bridges may occur. Additionally, under the fixed-ball-size requirement, solder balls must have a certain size, which in turn limits the number of solder balls that can be packed on the surface of a die.

In the other category of packaging, dies are sawed from wafers before they are packaged onto other wafers, and only “known-good-dies” are packaged. An advantageous feature of this packaging technology is the possibility of forming fan-out chip packages, which means the I/O pads on a die can be redistributed to a greater area than the die itself, and hence the number of I/O pads packed on the surfaces of the dies can be increased.

The bonding of dies to wafers includes dielectric-to-dielectric bonding (also referred to as fusion bonding), copper-to-copper bonding, and solder bonding. FIG. 1 illustrates a solder bonding scheme, wherein top die 100 is bonded onto bottom die 200 (which may be in a bottom wafer) through solder balls 106, which are placed between bond pads 104 and 204. A reflow is then performed to melt solder balls 106. In the case solder balls 106 are big enough, this bonding process is self-aligned, since the melted solder balls 106 may help the alignment of bond pads 104 with the respective bond pads 204, as is shown in FIG. 2.

In the case copper-to-copper direct bonding is performed, or in the case solder balls 106 are not big enough, there will be no self-alignment effect. Referring to FIG. 3, top die 100 is bonded onto bottom die 200 with bond pads 104 and 204 contacting each other directly, or bonded through very thin solder films (not shown). If bond pads 104 are misaligned with bond pads 204, the misalignment cannot be corrected by the subsequent reflow, if any. Therefore, each of the top dies has to be aligned to the bottom wafer accurately. This requires that the bonding of each of the top dies be accompanied by an alignment step. The throughput is thus significantly reduced.

In addition to the above-discussed problems, the conventional die-to-wafer bonding suffers from reliability issues. Referring to FIG. 4, after top dies 100 are bonded onto bottom wafer 300, molding compound 302 is filled into the gaps between top dies 100. Molding compound 302 is filled in the form of liquid, and is then solidified, for example, by annealing. During the filling of molding compound 302, bubbles 304 may be generated. Bubbles 304 may result in two consequences. First, in the subsequent die-thinning, bubbles 304 may cause the breakage of top dies 100. Second, if a vacuum is needed in the subsequent process, for example, when a chemical vapor deposition (CVD) step or a physical vapor deposition (PVD) step is involved, bubbles 304 may expand and hence result in damage to devices. New bonding methods are thus needed to improve throughput without incurring the above-discussed problems.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method of forming integrated circuits includes laminating a patterned film including an opening onto a wafer, wherein a bottom die in the wafer is exposed through the opening. A top die is placed into the opening. The top die fits into the opening with substantially no gap between the patterned film and the top die. The top die is then bonded onto the bottom die, followed by curing the patterned film.

In accordance with another aspect of the present invention, a method of forming integrated circuits includes providing a wafer including a plurality of bottom dies; and patterning a film to form a plurality of openings in the film. The plurality of openings has positions aligned to positions of the plurality of bottom dies. Each of the plurality of openings extends from one side to an opposite side of the film. The patterned film is laminated onto the wafer, with each of the plurality of openings in the film being aligned to one of the plurality of bottom dies.

In accordance with yet another aspect of the present invention, a method of forming integrated circuits includes etching a top surface of a top wafer along scribe lines of the top wafer; polishing a back surface of the top wafer to separate the top wafer into a plurality of top dies; and patterning a film to form a plurality of openings in the film, wherein the plurality of openings has positions corresponding to positions of a plurality of bottom dies in a bottom wafer. The film is laminated onto the bottom wafer, wherein each of the plurality of openings in the film is aligned to one of the plurality of bottom dies. Each of the plurality of top dies is placed into one of the plurality of openings. After all of the plurality of top dies are placed in the plurality of openings, the plurality of top dies is bonded onto the bottom wafer. The film is then cured.

In accordance with yet another aspect of the present invention, an integrated circuit structure includes a bottom die; and a top die bonded onto the bottom die, wherein the top die has a slanted edge. A filler material is on the bottom die and encircles the top die.

The advantageous features of the present invention include self-alignment resulting from accurate alignment of the patterned adhesive film with the bottom wafer, and hence quick pick-and-place may be performed. Top dies are automatically aligned to respective bottom dies. The throughput of the bonding process is thus significantly increased.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIGS. 1 and 2 illustrate a conventional solder bonding process, wherein top dies are bonded onto bottom dies by resorting to the self-alignment caused by the reflow of solder balls;

FIG. 3 illustrates a conventional direct copper-to-copper bonding process having a misalignment problem;

FIG. 4 illustrates a conventional step, wherein a molding compound is filled into the gaps between top dies; and

FIGS. 5-13 are cross-sectional views and top views of intermediate stages in a bonding process.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

A novel method of bonding dies onto a wafer is provided. The intermediate stages of performing a preferred embodiment of the present invention are illustrated. Throughout the various views and illustrative embodiments of the present invention, like reference numbers are used to designate like elements.

FIG. 5 is a top view of bottom wafer 20, on which (top) dies will be bonded. Bottom wafer 20 includes a plurality of bottom dies 22, which may be arranged, for example, in the style of an array with a plurality of rows and columns. The positions and sizes of bottom dies 22, particularly the relative positions of dies 22, are recorded for the formation of a patterned adhesive film, as will be discussed in subsequent paragraphs.

FIGS. 6, 7A and 7B illustrate the formation of patterned adhesive film 30. Referring to FIG. 6, a blank adhesive film 30 is provided. If viewed from the top, adhesive film 30 (refer to FIG. 7A) preferably has a substantially same size as bottom wafer 20 as shown in FIG. 5. Alternatively, blank adhesive film 30 may be larger or smaller than the size of bottom wafer 20. However, the size of adhesive film 30 is preferably great enough to cover all of the bottom dies 22. In the side view as shown in FIG. 6, blank adhesive film 30 appears as a sheet with a substantially uniform thickness T. In an embodiment, thickness T of the blank adhesive film 30 is close to the thickness T′ of top dies as shown in FIG. 12, although thickness T may be greater or smaller than thickness T′. Blank adhesive film 30 may be formed of a flexible material, which may be a polymer such as polyimide (including, for example, a thermosetting polyimide that can be cured by heat), a dry film, or the like. In alternative embodiments, blank adhesive film 30 is a hard, non-flexible film.

FIGS. 7A and 7B are a top view and a cross-sectional view, respectively, of adhesive film 30 after the patterning to form die-holding openings 32. Die-holding openings 32 are through-openings extending from one side of adhesive film 30 to the opposite side. Referring to FIG. 7A, die-holding openings 32 are accurately aligned to the positions of bottom dies 22 as in FIG. 5, so that when adhesive film 30 is placed over bottom wafer 20, bottom dies 22 may be exposed through die-holding openings 32.

FIG. 7B illustrates a cross-sectional view of patterned adhesive film 30, wherein the cross-sectional view is taken along a plane crossing line 7B-7B in FIG. 7A. In an embodiment, die-holding openings 32 have slanted sidewalls 33, with the bottom width W2 being smaller than the top width W1. In other words, top slant angles α of die-holding openings 32 are less than 90 degrees. The preferred slant angle α is greater than about 45 degrees and smaller than 90 degrees. The details of the top slant angles α are discussed in subsequent paragraphs. In an embodiment of the present invention, top slant angle α is about 54.7 degrees, although other greater or smaller angles may also be used, depending on the shape of the top dies that will be filled into die-holding openings 32.

FIGS. 8 through 10 illustrate the separation of top dies 42 from top wafer 40. FIG. 8 illustrates a cross-sectional view of top wafer 40, which includes a plurality of top dies 42. In FIG. 9, top wafer 40 is etched. In an embodiment, the etching is performed to the front surface of top wafer 40, wherein the front surface is the surface having bond pads 46 (refer to FIG. 12) thereon. Top wafer 40 is etched along scribe lines 44. The etching is preferably anisotropic, or has both anisotropic and isotropic components. In an embodiment, an anisotropic etch is first performed, then the etching recipe is adjusted to add an isotropic component in order to form slanted edges for dies 42 with slant angle β. The etchant recipe may be adjusted to form different slant angles β. However, slant angle β and slant angle α as shown in FIG. 7B are preferably equal to each other. This may be achieved, for example, by determining etchant recipes to perform experimental etchings in order to find out the respective slant angles β. If a slant angle β is desirable, then adhesive film 30 is formed to make slant angle α equal to the desired slant angle β. After the etching, the top width W2′ of top dies 42 is preferably equal to the bottom width W2 of die-holding openings 32 (refer to FIG. 7B), with as small a difference as possible.

Next, as shown in FIG. 10, glass wafer 50 (or a tape) is attached to the front surface of top dies 42, and the back surface of wafer 40 is grinded, until top dies 42 are separated from each other. The depth T′ of the resulting dies 42 is preferably, although is not required to be, close to the thickness T of patterned adhesive film 30 (refer to FIG. 6). In an embodiment, top dies 42 are sorted, and known-good-dies 42 are placed into a die tray (not shown). In alternative embodiments, top dies 42 are left in the dicing frame, in which the die-sawing or laser separation is performed, for the subsequent bonding process.

Referring to FIG. 11, patterned adhesive film 30 is laminated onto bottom wafer 20. An alignment is performed so that die-holding openings 32 are accurately aligned to bottom dies 22. As a result, bottom dies 22 are exposed through die-holding openings 32. The alignment may be performed by aligning the alignment marks on bottom wafer 20 with the alignment marks on patterned adhesive film 30. In an embodiment, the patterned adhesive film 30 itself is adhesive, and is reliably attached onto bottom wafer 20. In alternative embodiments, patterned adhesive film 30 is attached onto bottom wafer 20 through an adhesive.

Next, as shown in FIG. 12, a quick pick-and-place is performed to transfer top dies 42 into die-holding openings 32. This step is performed without any additional alignment step. Since the bottom width W2 of die-holding openings 32 (refer to FIG. 7B) is the same as the top width W2′ of top dies 42 (refer to FIG. 10), top dies 42 are self-aligned to respective bottom dies 22. Particularly, when the sidewalls of top dies 42 and die-holding openings 32 are slanted, the self-alignment is more accurate, and the pick-and-place is easier. Further, the size of top dies 42 preferably matches the size of die-holding openings 32 as much as possible, so that no gap is left between top dies 42 and adhesive film 30. When a face-to-face bonding is performed, the bond pads 46 in top dies 42 are accurately aligned to bond pads 23 in bottom dies 22. The bonding may be direct metal-to-metal bonding, or through a thin layer of solder.

After die-holding openings 32 are filled with top dies 42, a bonding is performed, during which an annealing may be performed, and a pressure may be applied to press top dies 42 against bottom wafer 20. Top dies 42 are thus bonded onto bottom wafer 20. In an embodiment, the annealing also causes the curing of the patterned adhesive film 30, and hence the patterned adhesive film 30 is solidified. In alternative embodiments, after the step of bonding, an additional curing may be performed, wherein the additional curing may include one or more steps of ultra-violet (UV) curing, heating, and the like, and combinations thereof. Advantageously, the cured patterned adhesive film 30 acts as the molding compound that otherwise would be filled into the gaps between top dies 42 in the liquid form.

Optionally, after the bonding of top dies 42 and the curing of the patterned adhesive film 30, a grind/polish step may be performed to level the back surface of patterned adhesive film 30 and the back surfaces of top dies 42. The resulting structure is shown in FIG. 13. It is noted that the back surfaces of top dies 42 may be higher than, substantially level with, or lower than, the top surface of patterned adhesive film 30. The polish may thus create a flat surface for subsequent process steps. In an alternative embodiment, no grinding/polish is performed, and the structure as shown in FIG. 12 may be sawed as is. FIGS. 12 and 13 illustrate the scribe lines 24, through which the bottoms dies 22, top dies 42 and the patterned adhesive film 30 are sawed apart. After the sawing, each of the resulting dies will include a bottom die 22, a top die 42, and a portion of adhesive film 30 on the bottom die 22 and encircling top die 42.

In above-discussed embodiments, direct metal-to-metal bonding or solder bonding (with little solder) may be used. However, it is realized that the embodiments of the present invention may be used to perform any other bonding techniques such as dielectric-to-dielectric bonding. In addition, one skilled in the art, with the teaching provided in the embodiments of the present invention, may realize the formation details of other bonding schemes, such as face-to-back bonding, back-to-back bonding, and the like.

The embodiments of the present invention have several advantageous features. By forming patterned adhesive film 30 with die-holding openings 32 accurately aligned to bottom dies 22 in bottom wafer 20, only one alignment needs to be performed to align patterned adhesive film 30 to bottom wafer 20, while top dies 42 may be quickly picked and placed into die-holding openings 32, which has the function of self-aligning top dies 42 to the respective bottom dies 22. The throughput is thus significantly improved.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

1. A method of forming integrated circuits, the method comprising: laminating a patterned film comprising an opening onto a wafer, wherein a bottom die in the wafer is exposed through the opening; placing a top die into the opening, wherein the top die fits into the opening with substantially no gap between the patterned film and the top die; bonding the top die onto the bottom die; and curing the patterned film.
 2. The method of claim 1, wherein the wafer comprises a plurality of bottom dies, the patterned film comprises a plurality of openings, and wherein after the step of laminating the patterned film, each of the plurality of bottom dies is exposed through one of the plurality of openings.
 3. The method of claim 1, wherein the step of laminating the patterned film comprises an alignment step to align the opening with the bottom die, so that after the step of placing the top die into the opening, the top die is self-aligned to the bottom die.
 4. The method of claim 1, wherein, between the step of placing the top die and the step of bonding the top die onto the bottom die, no additional alignment action is performed to align the top die to the bottom die.
 5. The method of claim 1, wherein the opening has slanted sidewalls, and the top die comprises slanted edges.
 6. The method of claim 5, wherein a first slant angle of the slanted sidewalls of the opening is substantially equal to a second slant angle of the slanted edges of the top die.
 7. The method of claim 1 further comprising, after the step of curing the patterned film, performing a polish to level a top surface of the patterned film with a back surface of the top die.
 8. The method of claim 1 further comprising, after the step of curing the patterned film, sawing the bottom die and the top die from the wafer, wherein no polishing step is performed to level a top surface of the patterned film with a back surface of the top die.
 9. A method of forming integrated circuits, the method comprising: providing a wafer comprising a plurality of bottom dies; patterning a film to form a plurality of openings in the film, wherein the plurality of openings has positions aligned to positions of the plurality of bottom dies, and wherein each of the plurality of openings extends from one side to an opposite side of the film; and laminating the film onto the wafer, wherein each of the plurality of openings in the film is aligned to one of the plurality of bottom dies.
 10. The method of claim 9 further comprising: placing each of a plurality of top dies into one of the plurality of openings; after all of the plurality of top dies are placed into the plurality of openings, performing a bonding step to simultaneously bond all of the plurality of top dies onto the wafer; and curing the film.
 11. The method of claim 10 further comprising: providing a top wafer; etching a top surface of the top wafer along scribe lines in the top wafer; and polishing a back surface of the top wafer to separate the top wafer into the plurality of top dies.
 12. The method of claim 10, wherein the step of curing the film is performed simultaneously with the step of bonding all of the plurality of top dies onto the wafer.
 13. The method of claim 10, wherein the step of curing the film comprises performing a heating step after the step of bonding all of the plurality of top dies onto the wafer.
 14. The method of claim 10, wherein, between the step of placing each of the plurality of top dies and the step of bonding all of the plurality of top dies onto the wafer, no additional alignment action is performed to align any of the plurality of top dies to a respective one of the plurality of bottom dies.
 15. The method of claim 9, wherein each of the plurality of openings has slanted sidewalls, and each of the plurality of top dies comprises slanted edges, and wherein, after the step of laminating, wider ends of the plurality of openings are farther from the wafer than narrower ends of the plurality openings.
 16. The method of claim 15, wherein a first slant angle of the slanted sidewalls of the plurality of openings is substantially equal to a second slant angle of the slanted edges of the plurality of top dies.
 17. A method of forming integrated circuits, the method comprising: etching a top surface of a top wafer along scribe lines of the top wafer; polishing a back surface of the top wafer to separate the top wafer into a plurality of top dies; patterning a film to form a plurality of openings in the film, wherein the plurality of openings has positions corresponding to positions of a plurality of bottom dies in a bottom wafer; laminating the film onto the bottom wafer, wherein each of the plurality of openings in the film is aligned to one of the plurality of bottom dies; placing each of the plurality of top dies into one of the plurality of openings; after all of the plurality of top dies are placed in the plurality of openings, bonding the plurality of top dies onto the bottom wafer; and curing the film.
 18. The method of claim 17, wherein the film is formed of a thermosetting soft material.
 19. The method of claim 17, wherein each of the plurality of top dies comprises slanted edges.
 20. The method of claim 17, wherein, before the step of bonding, no alignment action is performed to align any of the plurality of top dies to respective ones of the plurality of bottom dies. 